1. Technical Field
The present invention relates to a voltage controlled oscillator configured to control oscillation frequency by applying a voltage signal and, more specifically, relates to a frequency spread voltage controlled oscillator configured to output an oscillation signal while changing the oscillation frequency within short periods of time.
2. Related Art
Nowadays, for digital signal transmission, parallel transmission and serial transmission are both used. However, for transmitting a large volume of signals at high speed, serial transmission is mainly used. To serially transmit a digital signal, clocking is required as a reference for the transmission. As the amount of information to be transmitted increases, the transmission rate increases and the clock frequency becomes high. As the clock frequency is increased, high frequency signals at relatively high levels are continuously transmitted in response to the clock frequency. A problem has arisen in that these signals negatively affect peripheral devices as noise (electro-magnetic interference) (hereinafter referred to as EMI).
To solve this problem, various voltage controlled oscillators (VCOs) employing a spread spectrum method in which the clock frequency is changed over time and spread so as to prevent the noise being concentrated at a particular frequency has been discussed (for example, refer to Patent Documents 1 to 4).
Patent Document 1: Japanese Unexamined Patent Application Publication No. 9-98152
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-101408
Patent Document 3: Japanese Patent Application No. 2756739
Patent Document 4: Japanese Examined Patent Application Publication No. 8-10807).
The above-described VCOs, however, have various shortcomings. As a method of controlling the oscillation frequency (oscillation frequency), the above-described voltage controlled oscillators employ a method of connecting a varactor diode (variable capacitance diode) to a resonator element included in an oscillator circuit and applying a variable voltage signal whose amplitude changes over time to the varactor diode. In general, a triangular wave signal is used for the voltage signal.
The varactor diode has a characteristic as shown in FIG. 9.
FIG. 9 illustrates the characteristic of the applied voltage with respect to the capacitance of the varactor diode.
As shown in FIG. 9, the change rate of the capacitance of the varactor diode decreases as the voltage applied to the varactor diode increases, whereas the change rate of the capacitance of the varactor diode increases as the voltage applied to the varactor diode decreases. Consequently, when a triangular wave signal is applied, the change rate of the capacitance is lower in the sections of the triangular wave signal where the applied voltage is higher, that is, in the sections near the peaks of the triangular wave.
FIGS. 10(a)-10(c) illustrate the changes over time of the signal applied to the varactor diode, the capacitance of the varactor diode, and the oscillation frequency of the voltage controlled oscillator including the varactor diode, wherein FIG. 10(a) illustrates the applied voltage, FIG. 10(b) illustrates the capacitance of the varactor diode, and FIG. 10(c) illustrates the oscillation frequency of the voltage controlled oscillator. This drawing illustrates the characteristic of a case in which the cathode of the varactor diode is connected to a resonator element and the anode is grounded.
As shown in FIGS. 10(a)-(c), when a triangular wave signal is applied, the changes over time of the capacitance of the varactor diode and the oscillation frequency of the oscillator become gentle near one of the peaks of the triangular wave signal (for the capacitance, the peak on the high capacitance side and, for the oscillation frequency, the peak on the low frequency side) and become abrupt near the other peak (for the capacitance, the peak on the low capacitance side and, for the oscillation frequency, the peak on the high frequency side).
Therefore, when a triangular wave signal is applied, in the sections where the change of the oscillation frequency is gentle, there is a problem in that the frequency spread effect (EMI noise reduction effect) is insufficient.